1. Technical Field
The present invention relates to such a molding tool for a molding with a cylindrical core as is employed mainly as a vibration absorbing rubber bushing or a vibration absorbing mount for vehicles.
2. Background Art
As the vibration absorbing rubber bushing or the vibration absorbing mount for vehicles, there is known a cylindrical type in which an elastic rubber member is placed between a cylindrical core as an inner cylinder and an outer cylinder member or in which the elastic rubber member is arranged around the cylindrical core by omitting the outer cylinder member.
The molding tool for the molding with the cylindrical core such as that cylindrical vibration absorbing rubber bushing is generally constructed such that insert dies confronting each other vertically to mold the axial side faces of a molding are arranged in a cavity forming portion of upper and lower molds to be clamped, such that support pins are disposed at the central portion of at least one insert die or both the insert dies of the upper and lower molds thereby to support the cylindrical core upright by fitting the end portions of the cylindrical core on the support pins and by retaining the two ends of the same, and such that a cavity is formed around the cylindrical core when the upper and lower molds are clamped, so that a molding material such as rubber may be shaped by injecting and charging it into the cavity (as disclosed in Japanese Utility Model Publication No. 28020/1993 or in Japanese Patent Laid-open No. 276382/1995).
Here, the portions, as vertically protruding from the cavity, of the supported cylindrical core are fitted in the fitting hole portions formed in the central portions of the upper and lower insert dies so that their upper and lower ends are retained in retaining step portions of the support pins. However, the molding material, as injected into the cavity, will flow through the spacings between the end portions of the cylindrical core and the surrounding inner circumferences of the fitting hole portions so far that it enters the small spacings between the end faces of the cylindrical core and the retaining step portions thereby to cause the end face burrs.
If the end portions of the cylindrical core are so snugly fitted in the inner circumferences of the fitting hole portions as to reduce the spacings substantially to zero, however, the cylindrical core cannot be easily removed at the parting time after the molding operation, thereby to deteriorate the workability. If the cylindrical core is held in a completely fitted state, moreover, the air may not be completely discharged from the cavity. Especially when the air resides in the vicinity of the upper end portion, voids are left in the molding to make the molding defective.
In Japanese Patent Laid-Open No. 276382/1995, therefore, it has been proposed to dispose such a bushing in the inner circumference of the fitting hole portion of the insert die as can fit the predetermined size of the inner circumference of the end portion of the cylindrical core tightly, thereby to fit and support the end portion at the bushing.
Since the proposed bushing has the tightly or snugly fitting structure, however, there is required special means for releasing that tightness, e.g., knock-out means such as a spring or a push rod when the molds are to be opened. In addition, where the upper end portion of the cylindrical core is snugly fitted at the bushing especially in the fitting hole portion of the upper insert die, the highest portion of the cavity reserves the air, which is not completely discharged even by injecting and charging the molding material, so that the residual air makes the defect. This air defect can be normally eliminated if the air leaves the mating portions of the individual molds, but these mold mating portions may form the burrs.
If the charging pressure of the molding material to be injected and charged into the cavity is lowered, on the other hand, the burr is reduced, but the molding tends to be defected by the internally left air. Therefore, the injecting and charging pressure is so set as to cause the aforementioned problems as few as possible. However, the defect due to the residual air or the burr is not completely eliminated.
Where a plurality of cavity forming assemblies are provided in the molding tool provided with an injection apparatus for one molding material, on the other hand, the injecting and charging pressures into the cavities in the individual cavity forming assemblies are required to be constant for homogenizing the moldings.
In view of the description thus far made, the invention contemplates to provide a molding tool which is enabled to discharge the air reliably from the vicinity of the upper end portion of a cylindrical core and to eliminate the end face burrs. The invention further contemplates to provide a molding tool for a molding with a cylindrical core, in which the pressures for injecting and charging the molding material into individual cavities are held constant where a number of cavity forming assemblies are arranged.
According to an aspect of the invention, there is provided a molding tool for a molding with a cylindrical core, comprising: an upper mold and a lower mold adapted to be clamped directly or through an intermediate mold and including insert dies confronting each other vertically for forming the axial side faces of a molding with a cylindrical core, the inert dies of said upper and lower molds being individually provided with fitting hole portions for supporting the end portions of the cylindrical core set therein, said lower die being provided at its fitting hole portion with a support pin fitted in said cylindrical core from the lower end opening for retaining the lower end of said cylindrical core, said upper die being provided at its fitting hole portion with a core holder confronting said support pin concentrically and fitted on the upper end of said cylindrical core, said upper and lower molds being clamped to form a cavity around the outer circumference of said cylindrical core supported by said support pin and said core holder, and a molding material such as rubber being charged into said cavity so that it may be molded integrally with said cylindrical core. As the means for solving the aforementioned problems, the molding tool is characterized by comprising: a seal ring mounted in the fitting hole portion of the insert die on the side of said upper mold for fitting the outer circumference of the upper end portion of said cylindrical core while holding a slight air ventilation; a molding material injection hole leading from the upper face of said upper mold to said cavity; and an air and molding material relief hole leading from said cavity to the upper face of said upper mold.
According to this molding tool, when the molds are clamped by supporting the cylindrical core, the molding material such as rubber, as injected from the injection hole into the cavity, is charged around the cylindrical core from the lower portion to push the internal air upward and expel it through the relief hole, and the excessive molding material is also charged to the relief hole so that the air residing in the molding material is also simultaneously discharged.
On the other hand, the air, as might otherwise be reserved in the vicinity of the upper end portion of the cylindrical core, is discharged by charging the molding material from the small spacing between the cylindrical core and the seal ring which is fitted on the outer circumference of the upper end portion of the cylindrical core, so that it is hardly left.
If the vulcanization is performed in this state, moreover, the charging pressure of the molding material in the cavity does not rise excessively high so that no extrusion of the molding material occurs from the small spacing between the seal ring and the end portion of the cylindrical core thereby to prevent formation of the burrs.
In the aforementioned molding tool, said molding material injection hole and said relief hole in said upper mold are preferably disposed at positions of 180 degrees with respect to each other across the axis of said cavity. As a result, the molding material, as injected from the injection hole, flows around the two sides of the cylindrical core and fills up the entire cavity, and a portion of the molding material can be so released from the relief hole at the position opposed to the injection hole as to expel the air thereby to improve the air discharge.
The aforementioned molding tool is preferred to further comprise a core receiving seal member mounted in the fitting core portion of the insert die on the side of said lower mold for fitting the lower end portion of said cylindrical core while holding a slight air ventilation. As a result, the burrs can be satisfactorily prevented from being formed even at the lower end portion of the cylindrical core.
On the other hand, the aforementioned molding tool is preferred to further comprise a runner plate placed separably over the upper face of said upper mold and including a molding material injecting runner groove and an air and molding material relief groove formed in the lower face of said runner plate and connected individually with said injection hole or said relief hole in said upper mold. As a result, it is possible to perform smoothly and satisfactorily the action to inject and charge the molding material and the action of releasing the excessive molding material and the air.
In the aforementioned molding tool, moreover, said support pin can be arranged to move axially upward and to be supported at a fixed position by being biased downward by a return spring, furthermore to be pushed up at a parting time against the biasing force of said return spring with suitable push-up means. By this pushing-up action, the lower end portion of the cylindrical core can be easily separated from the fitting portion of the seal member thereby to separate the molds easily.
On the other hand, said core holder can be supported to move axially within a predetermined size range so that it can be biased axially downward by spring means to hold the cylindrical core by the biasing force of said spring means in a mold closing state supporting the cylindrical core and so that it can be moved axially downward by opening the molds to separate the upper end portion of the cylindrical core from the fitting portion of the seal ring. It means that the cylindrical core can be stably held, even if it has an error in the axial size, by the biasing force of the spring means. Moreover, the core holder is moved axially downward by the biasing force of the spring means as the molds are opened after the molding operation, thus the upper end portion of the cylindrical core is naturally separated from the fitting portion of the seal ring. As a result, the molding can be left on the lower side so that it can be easily parted.
In the aforementioned molding tool, a plurality of cavity forming assemblies including the insert dies confronting each other vertically, the support pin and and core holder are suitably arranged in said upper and lower molds, and the injection holes for injecting the molding material into the cavities of the individual cavity forming assemblies are suitably arranged equidistantly from the injection port of an injection apparatus. As a result, although the molding material is injected and charged into the numerous cavities from the injection port of one injection apparatus, the injection distances to the individual cavities can be equalized to homogenize the injection rates and the injection and charging pressures.
On the other hand, said cavity forming assemblies are preferably arranged on a common circle around the injection port of the injection apparatus, and the injection holes of the individual cavity forming assemblies are preferably arranged on a common circle and equidistantly from said injection port. As a result, the injection distances from the injection port to the individual cavities can be easily adjusted.
In the aforementioned molding tool, the relief holes of the individual cavity forming assemblies are preferably connected by the relief grooves of said runner plate so that they have communication with the outside of the mold side. As a result, the excessive molding material portions from the relief holes to the relief grooves can be so continuous as to facilitate their removing works after the molding operation.
In the aforementioned molding tool, on the other hand, the relief grooves of said runner plate preferably form a circle around the injection port of the injection apparatus and are preferably provided at the upper ends of the individual relief holes with projection forming portions for forming small protrusions. As a result, the excessive portions, as molded in the individual relief holes, can be extracted and removed without difficulty from the relief holes.
In the aforementioned molding tool, moreover, the injection holes and the relief holes of the individual cavity forming assemblies are formed around the injection port of said injection apparatus and on radial lines extending through the axes of said cavity forming assemblies. Then, the flow of the molding material from the charge to the relief holes can be smoothened to ensure the charging of the molding material and the discharging of the air thereby to reduce the defective moldings.